drm/panel: sitronix-st7703: Stop tracking prepared

As talked about in commit d2aacaf0 ("drm/panel: Check for already
prepared/enabled in drm_panel"), we want to remove needless code from
panel drivers that was storing and double-checking the
prepared/enabled state. Even if someone was relying on the
double-check before, that double-check is now in the core and not
needed in individual drivers.

One thing to note for st7703 is that it has a special "allpixelson"
debugfs file. When this file is written the driver hacks a
disable/unprepare and then a prepare/enable to try to reset the
panel. Potentially that might have been relying on the old booleans we
removed. It'll still "work" because of the checks in the core but it
deserves a comment. This debugfs file didn't appear to be particularly
safe to use even before this patch since it would cause a
disabled/unprepared panel to become prepared/enabled.

Cc: Guido Günther <agx@sigxcpu.org>
Cc: Ondřej Jirman <megi@xff.cz>
Cc: Chris Morgan <macromorgan@hotmail.com>
Cc: Frank Oltmanns <frank@oltmanns.dev>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Maxime Ripard <mripard@kernel.org>
Signed-off-by: Douglas Anderson <dianders@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20240503143327.RFT.v2.42.Ifc436b262d72f1a33ddef10adfd7578d4acb60d8@changeid